1. Field of the Invention
The present invention relates to a superconducting fault current limiter, and more particularly, to a resistive superconducting fault current limiter.
2. Description of the Conventional Art
In a power system comprising a power station, a power transmission line, a power transforming substation, a power distribution line, etc. for a power generation, a power transmission, a power distribution, etc., a fault current limiter (FCL) is a device for limiting a mechanical stress, a thermal stress, an electric stress applied to a bus-bar, an insulator, a circuit breaker, etc. at the time of an occurrence of a fault current due to a short circuit fault, a ground fault, etc. Since the fault current is continuously increased and it is difficult to develop a power device that is not influenced by the fault current, a demand for the fault current limiter is being increased. However, the development of the fault current limiter applicable to the power system has been delayed due to a technical difficulty and a commercial difficulty.
As a high temperature superconductor (HTS) was discovered, a fault current limiter using a non-linear voltage-current characteristic of the new device was developed, and a high temperature superconductor using liquid nitrogen as refrigerants has been earnestly developed since 1987.
The superconductor has a high non-linear resistance characteristic thereby to have a possibility to be applicable as a fault current limiter. However, an HTS using liquid helium as refrigerants has not been researched well due to an very expensive cooling cost. Recently, as an HTS using liquid nitrogen as refrigerants is developed, various superconducting fault current limiters using a superconductivity are being shown.
The superconducting fault current limiter uses a quench characteristic that a superconductor thereof is transited from a superconducting state to a normal conducting state. That is, the superconducting fault current limiter limits a fault current as a superconductor having a rapidly increased resistance serves as a fuse. When a value of the fault current is decreased, the superconductor of the superconducting fault current limiter is transited from the normal conducting state to the superconducting state. The superconducting fault current limiter has various types such as a resistive type, an inductive type, a hybrid type, etc.
Among said various fault current limiters, the resistive fault current limiter is the most developable and commercial.
The resistive superconducting fault current limiter has a simple structure, and has a lighter weight and a lower cost than the inductive type. However, when the resistive superconducting fault current limiter is operated, a hot spot where heat is generated is caused. Also, there is a difficulty in developing an excellent superconducting fault current limiter.
In order to solve the problem of the resistive superconducting fault current limiter, excessive heat partially generated at a fault current limiting device at the time of a quench phenomenon that a superconductor is transited from the superconducting state to the normal conducting state has to be dispersed, and the quench phenomenon has to be simultaneously generated at the fault current limiting devices connected to each other in serial in order to increase a voltage capacity of the fault current limiter.
In order to increase the voltage capacity of the fault current limiter, various methods such as a method for inserting a parallel resistance, a method for simultaneously causing the quench phenomenon by a proper serial or parallel combination, etc. have been researched. However, a certain solution has not been proposed.
Also, proposed was a method for inducing a uniform quench phenomenon among superconducting fault current limiters with using a heater to increase a temperature of a magnetic field and a superconducting device in a superconducting fault current limiter using Bi-2223 ring and rod type. However, said method does not solve an influence on a power system stability, an induction heat generated at a coil of a magnetic field applying unit as a current flows to the magnetic field applying unit, an influence of a reactance of the coil of the magnetic field applying unit on the system.
FIG. 1 is a construction view of a superconducting fault current limiter in accordance with the conventional art.
As shown, the conventional superconducting fault current limiter 1 comprises: a superconducting device 2, a resistive device; a non-metallic cryostat 4 filled with a refrigerant such as liquid nitrogen, for maintaining the superconducting device 2 as a superconducting state; a linear coil 6 for uniformly applying a magnetic field to the entire region of the superconducting device 2; and current leads 3 and 5 for connecting the superconducting device 2 and the linear coil 6 in serial.
The superconducting device 2 is a resistive device and is located in the cryostat 4. The cryostat 4 is filled with liquid nitrogen in order to cool the superconducting device 2 thereby to maintain the superconducting device 2 as a superconducting state.
The cryostat 4 is located in the linear coil 6 formed of a foil winding, and the linear coil 6 is connected to the superconducting device 2 in serial through the current leads 3 and 5. A current of a circuit of the power system flows in opposite directions each other through the current leads 3 and 5. The linear coil 6 is constructed so that the current uniformly applies a magnetic field to the entire region of the superconducting device. The linear coil 6 is a foil winding formed of copper or aluminum. The linear coil 6 horizontally supplies a magnetic field to the superconducting device 2, and is constructed to have a low inductance and a low magnetic field when a current is applied thereto.
A metal oxide varistor 7 is connected to the superconducting fault current limiter 1 in parallel in order to restrain an over-voltage.
Operation of the conventional superconducting fault current limiter 1 will be explained as follows.
When a fault current flows on a power system, a current flowing to the linear coil 6 is greatly increased. The increased current generates a magnetic field through the linear coil 6. The generated magnetic field is horizontally applied to the superconducting device 2, and the applied magnetic field exceeds a predetermined threshold value of a magnetic field of the superconducting device 2. By the magnetic field generated through the linear coil 6 and the increased current flowing to the superconducting device 2, the superconducting device 2 is transited to a resistive state. As the magnetic field generated through the linear coil 6 is uniformly applied to the superconducting device 2 horizontally, the quench phenomenon of the superconducting device 2 is uniformly caused at the entire region of the superconducting device 2. According to this, a partial heat generation of the superconducting device and a mechanical accident that a structure is destroyed as a mechanical force is generated due to a concentration of an electric field and a magnetic field can be prevented. Also, since the magnetic field is uniformly applied to the superconducting device 2 horizontally, a resistance generation ratio of the superconducting device 2 is increased. According to this, a development of a resistive fault current limiter having a fast responsiveness was possible.
In order to prevent a damage of the superconducting device due to heat, in the conventional resistive superconducting fault current limiter, the linear coil 6 is installed outside the cryostat, the linear coil 6 is connected to the superconducting device 2 in serial, and a technique for directly connecting the linear coil 6 to the power system is applied.
Therefore, in the conventional resistive superconducting fault current limiter, the quench phenomenon can be uniformly caused at the entire region of the superconducting device, and the quench phenomenon can be simultaneously caused at the superconducting devices connected to each other in serial.
However, in the conventional resistive superconducting fault current limiter, even when a normal current flows to the power system, a bad influence is afflicted on a magnetic field generated by a current flowing to the linear coil, and is afflicted on a stability of the power system by a reactance component of the linear coil. That is, even when a normal current flows on the power system, since the linear coil 6 is always in connected to the power system, a magnetic field is generated by the linear coil due to the current flowing to the linear coil, and the generated magnetic field influences on the superconducting fault current limiter.
Also, in the conventional resistive superconducting fault current limiter, since the current of the circuit flows to the resistive superconducting fault current limiter even under a normal operation, a current of a superconducting device of the conventional resistive superconducting fault current limiter can be lowered. According to this, the size of the resistive superconducting fault current limiter is increased, or the number of the superconducting devices of the resistive superconducting fault current limiter is increased. Said problems become serious as a capacity of the resistive superconducting fault current limiter becomes large with a high voltage. As the capacity of the resistive superconducting fault current limiter becomes large, the size of the linear coil and the number of windings have to be increased thereby to have a disadvantage in cost and operation.
In the conventional resistive superconducting fault current limiter, the current of the circuit of the power system, that is the current of an electric circuit between a power source and a load flows to the linear coil. According to this, heat generated at the linear coil has to be insulated.
Also, in the conventional resistive superconducting fault current limiter for inducing a simultaneous quenching to superconducting devices, it was possible to use a Bi2Sr2CaCu2Ox based (hereinafter, BISCCO group) superconducting device but there was a problem in using a Y—Ba—Cu—O based (hereinafter, YBCO group) superconducting device. The reason is because a horizontal magnetic field of the YBCO based superconducting device has to be greater than a vertical magnetic field thereof by several tens of times in order to generate the same amount of magnetic field. According to this, in the size and the number of turns of the linear coil, effectiveness and efficiency are degraded.